First question on my build

[J Luth]

No question that the 100R needs to be connected. The 100R is in the amp and connected it is just going through a switch. The reasons are there have been a few suggestions were I had to connect and disconnect the 100R, I figured having it on a switch would make it easier rather soldering it in and out or using a jumper. Also as eschertron suggested about doing a simulated real world test, I have to use the amp this weekend and I will do a real world test rather than simulated if it's still an issue. If there is to much hum to use the amp with the 100R connected I will be able to disconnect it by just flipping the switch rather than taking it apart to disconnect it.

[Dave H]

With the 100R in circuit the trace is at 5 divisions peak to peak, with the 100R lifted by the switch the trace drops to almost exactly 1 division peak to peak.

A 5x gain difference looks about right to me. I think it's as Chuck and Enzo said, there's still a hum issue which is more obvious with the resistor in place. The wire between the B+ diodes and first filter cap also carries high current charging pulses which can cause 120Hz hum. Where is that wire routed?

[J Luth]

Originally that wire was running down the side of the amp along with a the 6.3v from the PT to the pilot light and the power switches. It then connects to a terminal board along with the standby switch and then a wire to the filter cap. I moved the wire yesterday figuring it may be interacting with the 6.3v and pulled it up away from everything I could. It was late and had to stop before I could see if it did anything. Later when I get the bench cleared of what I am doing I'll try it routing it different ways

[J Luth]

Got some time today. I redid the grounding a bit, I had 4 different grounds on the chassis it's now 3, one for the tone caps, preamp and the 100R, one for the power supply section and the other is the line cord. Separated the bias supply and the rectifier, they were on the same board, and moved the rectifier down near the filter caps and made sure all the wires are not touching anything or close to any others. Before I connected the grounds to their current locations, I tried them in different configurations with jumpers to make sure it did not get worse. The hum did diminish a slightly. I have been looking for bad solder joints, anything visibly wrong or shorts. The only thing I found, not sure if it correct is if I measure the junction of the 820R and the 100R to the chassis I get a reading of 820 Ohms, if I measure the other side of the 820R where the wire connects that is connected to the 4 Ohm tap of the OT to the chassis it shows as a short.

[Chuck H]

The only thing I found, not sure if it correct is if I measure the junction of the 820R and the 100R to the chassis I get a reading of 820 Ohms, if I measure the other side of the 820R where the wire connects that is connected to the 4 Ohm tap of the OT to the chassis it shows as a short.

That is normal. The 820R is connected directly to the OT secondary that is a VERY low resistance. So the 820R resistor through the OT secondary to chassis is still pretty damn close to 820R. Then if you measure the other side of the 820R you're basically measuring the OT secondary resistance to chassis. Which, on many meters (if you don't take "null" into account), reads as a virtual short.

[dstrat]

from the photo in #134 looks like your using crimp ring connectors. maybe thats ok but there are i counted at least 6 or 7 wires connected to that bolt. not really optimal IMO, also you really should be using solderable toothed ring connectors to give some bite into the metal if not already.

[Dave H]

Originally that wire was running down the side of the amp along with a the 6.3v from the PT to the pilot light and the power switches. It then connects to a terminal board along with the standby switch and then a wire to the filter cap. I moved the wire yesterday figuring it may be interacting with the 6.3v and pulled it up away from everything I could. It was late and had to stop before I could see if it did anything.

Did moving that wire have any effect on the 120Hz hum? It looks like it's running in in a long loop.

[J Luth]

Thanks Chuck

[J Luth]

That's all been changed. All of those crimp connectors were soldered also. Still trying different grounding.

[J Luth]

I moved the rectifier down by the filter caps, the wire from the rectifier to the cap is now only about 3" long but the wire from the standby switch is still long.
What I have done has had a small effect on the hum but I really haven't had a chance to check it much. The past couple of days has been hectic here doing quick in and out jobs for people. That's what I get for doing 1 day or same day turnaround's.

I'll be using it tomorrow, well actually tonight. That will be a real world test.

[Dave H]

I'll be using it tomorrow, well actually tonight. That will be a real world test.

We'll expect a full report

[J Luth]

Did a bit more cleaning up inside the amp but I didn't get a chance to try the amp other than turning it on and making sure it worked before I had to head out tonight.

Well the full report would be, I took it to the gig tonight, no mushroom cloud, I didn't let the smoke out, the other members said the amp sounded better than it did and they liked it before all this and dare I say it, not a peep out of it, no hum, no buzz no nuttin just tons of volume and tone. The only issue with it is it now has a ton of gain, Have to pay close attention to the volume knob. It was a small club didn't need the volume up much, probably in the area of a little more than 1/2 way between off and 1/4 and it was considerably louder than it would have been before at that same setting.

I have a couple of more coming up in a week or so only time will tell.

So if it is all set, time will tell, I would like to thank all that have helped with this.

Now to figure out the output watts.

So here's another question or two.
Is putting a signal into the amp, taking an ACV reading at the output jack, squaring that number and dividing it by the OHM load a close to accurate way to get the output? If so what would be the best tone to use. I have the small oscillator that can generate the tone and has an amplitude knob but not sure which tone to one use and what to set the amplitude of the oscillator to. Or is there a better way to get the output with what I have for equipment which includes an oscilloscope.

[g1]

The trick to measuring the power out is knowing when the amp distorts. Once the amp hits full power and starts clipping, the output power measured will increase fast. But you are really interested in clean power.
Since you have an o-scope, it's easy to see. Set the amp volume to 1/3rd or 1/2, tone controls mid way. Bring the test tone amplitude up from zero til you see the sine wave start to flatten out on top and bottom. Back off till the flattening is just barely starting. Now measure the AC volts at the load and do your calculation. My preferred test tone frequency is 400Hz, many use 1Khz.

[J Luth]

Thanks g1 just saw this.

I was messing with it today with the scope and meter and was doing it close to what you wrote. I started with 1Khz and also tried a few others other difference was I set the oscillator amplitude to about 1/4 tone controls in the center and turned the amp volume knob up until the peaks of the wave started to flatten. Then fine tuned the volume knob until it was just before the wave flattened.
Wasn't sure what tone to use or volume settings. I definitely need to mount the small dummy load to a heat sink or get a 100 - 200 watt dummy load, gets hot quickly.

[g1]

So what kind of power output did you get?